1. Field of the Invention
The present invention generally relates to materials with reduced bulk and surface defects and more particularly to GaN layers for optoelectronic materials, electronic materials, and other materials with reduced defects.
2. Background Description
Gallium nitride and its alloys with InN and AlN have recently emerged as important semiconductor materials with applications to yellow, green, blue and ultraviolet portions of the spectrum as emitters and detectors, and as high power/temperature electronics. Estimates are in the billions of dollars per annum for business activity surrounding nitride semiconductor based light emitters and to some extent power devices.
GaN and related heterostructures, however, suffer from a large concentration of structural and point defects. This is due to lack of native substrates being available. The most commonly used substrate is sapphire. There is a large lattice and thermal mismatch between GaN and sapphire. To circumvent this, a process of called xe2x80x9cLateral Epitaxial Overgrowthxe2x80x9d is utilized in many instances. This process is imperative in lasers with long longevity. This process requires a growth sequence to be completed. Then the wafer is removed from the reactor, patterned with SiO2 or Si3N4 and put back in the growth vessel for the continuation of the growth. The post pattern growth process is tailored to promote lateral growth followed by vertical growth after complete coalescence. During lateral growth, the area above the dielectric mask grows out and merges with the one from the other side. Fundamentally, that region of the material will have structural defects unless the GaN below the dielectric mask is coherent. A schematic representation of this process is shown in FIG. 1. As can be seen in FIG. 1, there is shown a cross-sectional view of a portion of a wafer. A GaN layer 1 over a substrate 2 has a SiO2 dielectric mask 3. GaN epitaxy 4 is grown over dielectric mask 3. Defects form in lateral growth wings 5 over the dielectric mask 3. There are still many technological and fundamental problems with this approach and the method is viewed as temporary.
Another approach that has been explored is to grow very thick, 200-300 xcexcm GaN layers on sapphire by hydride Vapor Phase Epitaxy, remove the GaN layer from the substrate, polish both sides, and fine polish to render the top face epitaxy worthy. With process conditions that have not been reported, one laboratory was able to reduce the extended defect concentration near the top surface. However, the seemingly same approach has not yielded the same success in other laboratories. In any case, this too is a rather involved process.
Nitride semiconductors have been deposited by vapor phase epitaxy (i.e., both hydride VPE (HVPE) which has been developed for thick GaN layers and organometallic VPE (MOVPE) which has been developed for heterostructures), and in a vacuum by a slew of variants of molecular beam epitaxy (MBE).
Nitride-based light emitting diodes (LEDs) with lifetimes approaching 100,000 hours (extrapolated) and brightness near 70 lm/W in the green have been obtained. These LEDs are already being used in full color displays, moving signs, traffic lights, instrumentation panels in automobiles and aircraft, airport runways, railway signals, flashlights, underwater lights. The technology is in the process of being extended to standard illumination under the name xe2x80x9cSolid State Lightingxe2x80x9d (SSL). SSL is expected to result in substantial energy savings by as much as a factor of six compared to standard tungsten bulbs. Along similar lines, blue lasers are being explored as the read and write light source for increased data storage density for the next generation of digital video disks (DVDs). Already, the room temperature CW operation in excess of 10,000 hours has been reported. To be versatile, this level of lifetime with a power level of about 20 mW at 60 C. is required. The present device lifetimes under these more stringent operating conditions are near 400 hours which is a long way from the needed 10,000 hours.
The large bandgap of GaN with its large dielectric breakdown field, coupled with excellent transport properties of electrons and good thermal conductivity, are well suited for high power electronic devices. Already, high power modulation doped field effect transistors (MODFETs) with a record power density of about 10 W/mm in small devices, and a total power of 8 W in large devices have been achieved. In addition to high power, and high frequency operation, applications include amplifiers that operate at high temperature and other unfriendly environments, and low cost compact amplifiers for earthbound and space applications.
When used as UV sensors in jet engines, automobiles, and coal burning furnaces (boilers), GaN-based devices will allow optimal fuel efficiency and control of effluents for a cleaner environment. Again, this is a direct result of the large bandgaps accessible by nitrides, as well as their robust character. GaN/AlxGa1xe2x88x92xN (from now on denoted GaN/AlGaN) UV pin detectors have demonstrated sensitivities of about 0.20 A/W or higher, and speed of response below a nanosecond.
Despite this progress, the defect concentrations of both structural and point defects are still high. This is mainly attributed to the lack of native substrates. To circumvent this somewhat, a flurry of activity has been expended on lateral overgrowth methods to block dislocations. However, if and when the base layer lacks long range coherence, the overgrown layer will naturally lack that coherence making it rather doubtful that a defect free material will emerge where the lateral growth fronts meet. Nevertheless, lasers with long longevity could be obtained only by this process as the overall structural defect density is reduced, primarily above the masked regions, by several orders of magnitude to around 107-108 cmxe2x88x922 from about 1010 cmxe2x88x922.
For electronic devices to hold promise in a given semiconductor, carrier mobility is generally used as a figure of merit. In addition, the carrier mobility is also used to deduce information regarding scattering centers and processes involved. GaN is no exception and consequently electron mobility in samples prepared by various methods has been a subject of discussion. In this vein, room temperature electron mobilities for MOVPE grown silicon doped GaN layers are typically reported to be in the range of 350-600 cm2Vxe2x88x921Sxe2x88x921, whereas that reported for hydride vapor phase epitaxy (in several tens of microns thick layers) is about 800 cm2Vxe2x88x921 sxe2x88x921. The highest room temperature mobility ever reported for GaN was 900 cm2Vxe2x88x921 sxe2x88x921 deposited by MOVPE, which has not been confirmed, for a 4 xcexcm thick layer. In contrast, the highest room temperature mobility for plasma-MBE grown GaN is around 300 cm2Vxe2x88x921 sxe2x88x921 on sapphire substrates and 560 cm2Vxe2x88x921 sxe2x88x921 on SiC for ammonia-MBE on sapphire is about 550 cm2Vxe2x88x921 sxe2x88x921 in 2 xcexcm thick layers. More recently, a combination of lateral epitaxial overgrowth by MOCVD and subsequent growth by RF MBE method has resulted in relatively high electron mobilities in GaN, approaching 800 cm2Vxe2x88x921 sxe2x88x921. The MOCVD grown layers are several microns thick whereas the MBE grown layers are thin and grown at growth rates in the low tenth of a micron per hour range. These figures compare with earlier predictions, which seemed to have converged around 900 cm2Vxe2x88x921 sxe2x88x921. Recently, these predictions had to be revisited as the room temperature mobility in modulation doped AlGaN/GaN structures began to approach about 2,000 cm2Vxe2x88x921 sxe2x88x921. Electron mobilities limited by polar optic phonon scattering have been predicted by Ridley to be 2200 cm2Vxe2x88x921 sxe2x88x921 for an electron effective mass of m*=0.22 m0.
The present invention is directed to reducing defects emanating from an underlying material when subsequent layers are added over the underlying material. The invention positions a layer of islands over the underlying material. A barrier layer is then positioned over the layer of islands. The layer of islands and the barrier layer may be repeated a plurality of times. The islands act to separate the barrier layer from the underlying material or the preceding barrier layer and thus reduce the defects of the underlying material from propagating to subsequent barrier layers. In this way, the island layers and the barrier layers make up a defect filter that acts to xe2x80x9cfilterxe2x80x9d the defects in the underlying material. After the appropriate number of alternating island layers and barrier layers have been applied, a final layer may be positioned over the filter.
Accordingly, the present invention includes a material having reduced bulk and surface defects. The material includes a base material having a first surface wherein the first surface has surface defects. A defect filter layer is positioned over the first surface. The defect filter layer includes alternating layers of islands and a barrier layer. The defect filter layer provides a second surface that has a reduced number of defects relative to said first surface. The islands and the barrier layer preferably have different lattice constants. The islands may be made of an island material selected from the group consisting of GaN, AlN, AlGaN, InGaN, and combinations thereof. The island materials and the barrier layer are preferably different from one another and may be selected from the group consisting of GaN, AlN, AlGaN, InGaN, and combinations thereof.
The present invention also includes a GaN material that includes a substrate, a buffer layer on one side of the substrate, and a defect filter layer positioned over the buffer layer. The defect filter layer has a thickness sufficient to reduce defects. Further, the defect filter layer includes at least one layer having a plurality of GaN islands. The defect filter layer preferably includes alternating layers of GaN islands and a barrier layer that has a lattice constant different than GaN. The filter layer material may be selected from the group consisting of AlN, InGaN, AlGaN, and alloys thereof. The number of alternating layers in the filter layer preferably ranges from about 1 to about 50. The defect filter layer preferably has a layer of GaN islands adjacent to the buffer layer.
The buffer layer may include alternating layers of GaN and a material having a lattice constant different than GaN. Further, the material may be a semiconductor. The material may be selected from the group consisting of AlN, InGaN, silicon, AlGaN, and combinations thereof. The number of alternating layers in the buffer preferably ranges from about 1 to about 10. The initial buffer layer preferably begins with a material having a lattice constant different than GaN and ends with a material having a lattice constant different than GaN. The substrate may be selected from the group consisting of sapphire, SiC, ZnO, GaAs, and silicon, and other oxide based substrates such as LiAlO2 and LiGaO2.
The present invention also includes a method for making a GaN material. The method includes growing a buffer layer over a substrate and growing a defect filter layer on the buffer layer to a thickness sufficient to reduce surface defects. The defect filter layer includes a plurality of GaN islands. The step of growing a buffer layer includes growing alternating layers of GaN and a buffer layer material that has a lattice constant different than GaN. The buffer layer material may be selected from the group consisting of AlN, InGaN, silicon, AlGaN, and combination thereof. The step of growing alternating layers in the buffer layer includes repeating the alternating layers of GaN and a material having a lattice constant different than GaN from about 1 to about 10 times. Preferably, the step of growing the buffer layer begins with growing an initial buffer layer of the material having a lattice constant different than GaN and ends with growing a final buffer layer of a material having a lattice constant different than GaN.
The step of growing the defect filter layer includes growing alternating layers of a plurality of GaN islands and a barrier layer having a lattice constant different than GaN. The step of growing alternating layers includes repeating growing alternating layers of a plurality of GaN islands and a barrier layer having a lattice constant different than GaN from about 1 to about 50 times. The step of growing the defect filter layer may include initially growing a layer of GaN islands on the buffer layer and ending the defect filter layer with a barrier layer having a different lattice constant than GaN. The barrier layer may be selected from the group consisting of AlN, InGaN, AlGaN, and alloys thereof. The substrate may be selected from the group consisting of sapphire, SiC, ZnO, GaAs, and silicon, and other oxide based substrates such as LiAlO2 and LiGaO2.
Still further, the present invention includes a semiconductor nitride layer. The semiconductor nitride layer includes GaN islands formed on a material selected from the group consisting of AlN, InGaN, silicon, AlGaN, and combinations thereof. The nitride layer has a surface layer of GaN covering the GaN islands and material.
It is also one object of the present invention to provide a thin GaN layer (about 1 micron) grown on a substrate by molecular beam epitaxy and utilizing a combination of ammonia and RF nitrogen sources to produce a surface with minimal structural defects.